WO2010060865A1 - Sinterable semi-coke powder with high bulk density - Google Patents

Abstract

In a continuous method for producing a semi-coke powder with a high bulk density and high flowability, a sinterable carbon powder (semi-coke) is dispersed in water while adding at least one binder and at least one fluidizer, wherein the carbon fraction of the dispersion is set to at least 50 wt % relative to the mass of the dispersion, and the zeta potential of the dispersion is set to less than -50 mV, the dispersion is subjected to a homogenization and stabilization through continuous wet grinding at a dwell time in the mill of less than 3 minutes, and the homogeneous dispersion is spray-dried.

Description

 Semi-coking powder with high bulk density

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

The invention relates to a high bulk density sinterable semi-coke powder, a continuous, fast process for its production, its use for the production of carbon and graphite ceramics and a slip which is an intermediate in the claimed process.

BACKGROUND OF THE INVENTION

Carbon and graphite ceramics with particle sizes below 20 microns are usually made of two-component systems with a solid (coke) and a binder. To improve the property profile additives are common, which increase the coking residue of the binder. Two-component materials have the fundamental disadvantage that after molding during carbonization, the coke used expands, while the solidifying binder framework shrinks. Only when the pretreatment temperature of the coke is exceeded, a shrinkage of both components takes place. This fundamental problem explains the formation of microcracks in the structure and the limitation of the achievable with such technology strengths in the finished ceramic. A new approach was taken by the use of sinterable Halbkoksen as CARBOSINT ^®, their preparation, in EP 0552422 Bl is described. Semi-coke is a carbon material that is an intermediate between a fusible pitch and an infusible green coke. It can be obtained by incomplete carbonization at a temperature between the onset of melting of the pitch and 500 ⁰ C, optionally in combination with an oxidation at below 250 ⁰ C. Optimally adjusted cokes act at the same time as filler coke and binder in the ceramic production. Due to the sticky surface, such a semi-coke powder can be processed without further additives by swaging or isostatic pressing. Despite the long availability of sinterable semi-cokes and their advantageous property profiles, these have

One-component materials so far not enforced in the market for the reasons explained below.

The advantage of the stickiness of the coke for its compaction means at the same time that such

Material is difficult to handle because it tends to clump and stick in the feeding systems to automatic presses or filling molds. Therefore, a homogeneous mold filling is considerably more difficult and associated with considerable effort.

Another disadvantage of this coke is the low bulk density of about 0.4 g / cm ³ . As a rule, a component based on fine-grain carbon has a geometric density of 1.2 to 1.3 g / cm ³ after shaping, which means that at least three times must be compacted during die pressing. Technically controlled is a Doubling the density. Higher compression factors limit the complexity and design freedom of the producible components - even in multi-level axial presses.

In the manufacture of components by isostatic pressing, the high tackiness of the coke makes it difficult to homogeneously fill the flexible mold. However, density gradients in the component resulting from inhomogeneities during filling can no longer be corrected in the further processing process.

These disadvantages still prevent many applications that require near net shape shaping. The mechanical finishing is usually too expensive. Thus, the full use of existing in sinterable semi-coke potential is prevented with respect to the product properties of technical carbon products.

To overcome these disadvantages and thus to

The preparation of sinterable semi-coke powder for the near-net shape production of carbon and graphite ceramics is proposed by US Pat. No. 4,985,184, the carbon powder with binder in an amount of from 0.01 to 5% by weight and a nonionic wetting agent in an amount of from 0.01 to 3 % By weight, based in each case on the mass of the solid, and then granulating by spray drying. Methylcellulose or hydroxycellulose are suggested as binders, and alkylphenol ethylene oxides as wetting agents. A disadvantage of this

The method is the low solids content of the suspension of only 33%, which has a high water evaporation performance in the Requires spray granulation, which is therefore associated with high process costs. Further disturbing in the further processing are the high additive amounts of up to 8 wt. %.

SUMMARY OF THE INVENTION

The invention is therefore based on the object to provide a sinterable Halbkokspulver with a significantly increased bulk density and flowability, which allows easy processing of the coke by pressing process, so that conventional dosing techniques can be used in the production of ceramic moldings.

This task is solved by a continuous process, in which one can

- a sinterable carbon powder (semi-coke) dispersed in water with the addition of at least one binder and at least one condenser, wherein the carbon content of the dispersion to at least 50 wt.%, Based on the mass of the dispersion, and the zeta potential of the dispersion to less than -50 mV stops,

subjecting the dispersion to homogenisation by continuous wet grinding with a residence time in the mill of less than 3 minutes, preferably less than 2 minutes, and

- The homogeneous suspension spray-dried.

The invention furthermore relates to the dispersion

(Slip), which is available in which one - A sinterable carbon powder dispersed in water with the addition of at least one binder and at least one condenser, wherein adjusting the carbon content of the dispersion to at least 50 wt.%, Based on the mass of the dispersion, and the zeta potential of the dispersion to at least -5OmV and

subjecting the dispersion to homogenisation by continuous wet grinding with a residence time in the mill of less than 3 minutes, preferably less than 2 minutes.

The invention further relates to the solid obtained by spray-drying in the form of granules.

During spray drying takes place as a result of a fast

Drying process atomized droplets of the dispersion agglomeration of the primary particles to much larger free-flowing secondary particles. The solid particles obtained after spray-drying have an angle of repose of 18 ° to 23 °, preferably 18.5 ° to 22.5 °, measured in

Based on DIN 53468. The non-granulated material has an angle of repose of 41 °. The d ₅₀ value of

Grain size distribution of the spray-granulated product was determined using a laser diffraction spectrometer from Malvern (Mastersizer 2000) and is 60 to 100 μm. This means that 50% by weight of the carbon particles have a diameter of 60 to 100 μm after spray-drying. This is the secondary particle size.

Finally, the invention relates to the use of the spray-dried carbon particles for the production of carbon and graphite ceramics. By the combined method of continuous wet dispersion with short residence times in the mill and spray granulation is a sinterable Halbkokspulver, recordable as the CARBOSINT ^® - powder according to EP 0552422 Bl, improved in its operability. Nearly ideal, round, flawless granules are obtained (FIG. 2). If the spray granulation is carried out without prior wet dispersion, the solid sedimentation of the solid in the spray-drying receiver vessel occurs despite the addition of additives and blockage of the spray nozzle. Also the

Bending fracture strengths inventively obtained test specimens are significantly higher than those of US 4,985,184 at 65 MPa.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows the dependence of the zeta potential of

Semi-coke particles from the pH of the suspension. 2 shows a scanning electroscopic image of a semicoke treated according to the invention.

Fig. 3 shows a rasterelektroskopische receiving CARBOSINT ^®.

Fig. 4 shows the bulk / tamped density of the product according to the invention as a function of the solids content.

Fig. 5 shows debris / tamped density as a function of the flow through the spray tower at a solids content of the slurry of 55.8% m / m

Fig. 6 shows the density of compacts as a function of the solids content of the slurry.

Fig. 7 shows the density of sintered compacts as a function of the solids content of the slurry. Fig. 8 shows the density as a function of the delivery rate in the spray drying at a given slip composition and evaporation performance.

Fig. 9 shows the density of a fired at a temperature of up to 1100 ⁰ C compact as a function of the flow rate in the spray drying.

Starting material for the process according to the invention are sinterable carbon powders (semi-cokes). The preparation of such a semi-coke powder is described in EP 0 552 422 B1, the disclosure of which is referred to for the purposes of this invention. Such a carbon powder is prepared by distilling a bituminous material, such as tars or coal tar or pitches, at a pressure of up to 5 mbar and a final temperature of at least 400 ⁰ C, dry grinding the distillation residue to the desired particle size and oxidation of the ground distillation residue at Temperatures below the temperature of incipient powder coating until the core of the individual particles no longer melts.

The sinterable Halbkokspulver is dispersed in water (slip). The proportion of the semi-coke powder in the dispersion is adjusted to preferably 55 to 59% by weight, based on the mass of the dispersion. The height of the solids content in the dispersion will depend on the further processability by spray drying. If the solids concentration is too high, stable spray drying is not possible. In general, however, the proportion will be set so high that the dispersion obtained can be further processed by spray drying. As dispersing additives, liquefiers and binders can be used. As condenser alkali and erdalkalifreie surfactants such as the commercially available substances Dolapix ligninsulfonate a manufactured with formaldehyde polymer and the naphthalenesulfonic acid (NSF) are preferably ⁸⁹ CA, Dolapix ET ^® 85 Trusan ^* 450, 470, 480 and 490 and used. Particularly preferred is compared to natural products with varying compositions synthetically produced NSF because of its reproducible molecular weight distribution. The proportion of the liquefier, based on the mass of the solid, may be 0.5 to 5 wt.%.

Suitable binders include polyethylene glycols having molecular weights of from 500 to 20,000 g / mol, preferably from 2,000 to 6,000 g / mol, and also polyvinyl alcohols. A suitable polyvinyl alcohol is, for example, PVA 4-88. The proportion of the binder, based on the mass of the solid, 0.5 to 2.5%, preferably 1 to 2 wt.%, Amount.

To obtain the desired stability of the slurry, the zeta potential is set to less than -45 mV, preferably about -50 mV. The pH of the slurry is adjusted to at least 7, preferably 8 to 12.

In order to ensure the desired stability of the slip, the slip dispersion can be wet-ground in a subsequent step in a continuous flow-through ball mill. In this case, a deagglomeration of the particles takes place with simultaneous wetting of the individual particles through the aqueous phase. Deagglomeration can also be assisted by treating the dispersion with ultrasound. Surprisingly, the residence time in the ball mill can be kept very short. This prevents a significant change in the particle size distribution. The mill acts predominantly as a dispersant with a high energy input.

The spray drying which follows the dispersing / milling process is a process which can be carried out continuously for drying solutions, suspended particles and pasty masses. By means of a nozzle (operated by liquid pressure or compressed air or inert gas) or rotating atomizer discs (4,000-50,000 rpm), the material to be dried is atomized into droplets in a hot air stream (temperatures up to 300 ^° C. depending on the apparatus) If dried by the spray tower to a fine granules of spherical shape. Depending on the design or intended use, the hot air can flow in cocurrent or in countercurrent to the spray jet. The resulting dry material is usually separated by a cyclone from the air flow and can be removed there. The particles obtained by the spray drying / granulation are free-flowing and have a size distribution d50 of 60 to 100 μm.

The bulk density of the spray granules according to the invention is 0.5 to 0.6 g / cm ³ ; the tamped density is about 0.6 g / cm ³ .

The following examples serve to further illustrate the invention.

Examples

Example 1 - Production of the sinterable semi-coke powder A coal tar pitch having a softening point (Mettler) of 50 ⁰ C, a content of quinoline-insoluble (QI) to DIN 51921 of 13.2 wt.% By weight and a content of toluene-insoluble (TI) in accordance with DIN 51906 of 32.7.% Is in one

Agitator evaporator distilled at a pressure of 1 mbar to 460 ⁰ C. The distillation residue has a melting point (Tottoli) of 350 ⁰ C and contains 57.4 wt.% QI, 91.9 wt.% Of Ti and 10.9 wt.% Volatiles. It is ground in a vibratory mill dry to a mean particle size of 30 microns. The powder begins to cake at about 28O ⁰ C. It is an oxidation temperature of only 18O ⁰ C chosen to prevent caking of the powder with the expected exothermic reaction with safety ^■ .

A portion of the ground residue is oxidized in the presence of air at 180 ⁰ C, taking one sample at intervals of one hour. The samples are pressed at 30 MPa into test specimens which are heated under inert gas in a chamber furnace with a temperature gradient of 1 K / min up to 1000 ⁰ C.

After an oxidation time of 10 hours no more swelling of the sample occurs. The required oxidation time is therefore 10 hours in order to make the powder infusible into the core of the individual particles. The remainder of the ground residue is therefore oxidized at 180 ^° C. for 10 hours in air. The oxidized powder has a softening start of 250 ^° C., an oxygen content of 2.9% by weight and an optical anisotropy of 15% by volume. It contains 70.9% by weight of QI, 94.5% by weight of TI and 0.1% by weight of volatile constituents.

Example 2 - Preparation of the carbon dispersion (slip)

The composition of 18 experimental samples is shown in Table 4.

The slip is made up of 3,000 g of CARBOSINT *,

2.375 g of deionized water, 60 g of NSF as condenser and 60 g of binder (PEG 4000) in the stirred reactor. The solid with a particle size distribution d ₅₀ of 8 to 9 μxn is slowly added in order to achieve the most uniform possible mixture. After a dispersion time of 1 h, the pH is adjusted to pH 10 with 25% strength ammonia solution and homogenized for a further 24 h. Subsequently, a fine dispersion (homogenization) is carried out in a bead mill type KDL Pilot ^® DynoJYIill of Willy A. Bachofen (WAB) AG. The grinding balls are made of cerium stabilized zirconia and have a diameter of 1.2 to 1.7 mm. The mill is filled to a degree of filling of 85% with grinding balls. The peripheral speed of the stirring discs is 10 m / s. The volume flow to be pumped through the mill by means of the displacement pump is 700 mL / min. The residence time in the bead mill is 50 to 80 seconds. After passing through the bead mill, the suspension is pumped into the spray tower, which is heated to the drying temperature.

Example 3 - Spray drying

The slip obtained in Example 2 is spray-dried with a DC operated spray tower of the company Nubilosa of the type LTC. This spray tower is a technical spray tower for maximum water evaporation capacity of 7.5 kg / h

Drying air temperature of 350 ⁰ C. The electric air heater has a heating capacity of 12 kW. The cylindrical part of the tower is 4 m long and has a diameter of 800 mm. At the upper end of the spray tower, the aqueous dispersion to be dried is introduced into the drying space via an externally mixing two-substance nozzle with a diameter of 1.5 to 2 mm. For atomization, a compressed air consumption of about 3 Nm ³ / h at a compressed air pressure of 3 bar is needed. At the lower end of the drying tower are a cyclone and a jet filter for separating the fines from the drying air stream. The product is collected at the lowest point of the apparatus. Suspension throughputs of 2 to 8 L / h at drying gas temperatures of 180 to 275 ⁰ C were driven.

The spray granulation conditions for 18 experimental runs are shown in Table 5.

Example 4 - Examination of debris and tamped density

Bulk density according to DIN 51705 and tamped density (tamped density) according to DIN 51916 were determined. The achievable debris and

Puncture densities increase with increasing solids content (FIG. 4) and pass through a maximum with increasing delivery volume (FIG. 5) through the spray tower. The determined bulk densities are 0.5 to 0.6 g / cm ³ . The tamped density according to DIN 51916 achieves values of a maximum of 0.61 g / cm ³ . The investigations, the results of which are shown in FIGS. 4 and 5, show that the densities of the molded articles sink with increasing solids content or increasing flow through the spray tower.

In addition to the processibility in presses, where a high bulk density is required, and the properties of the components must be optimized, it may be advantageous to adjust the solids content not above 58 wt.% And not below 56 wt.%, Each based on the mass of the slip.

Example 5 - Density of the compact

In the further processing of the powder (spray granules) in presses, the powder is pressed into shaped bodies and then fired. Decisive features for pressed or fired bodies are the achieved densities after pressing or firing and the bending strength after firing. Therefore, the densities of the pressed or fired shaped body were determined as a function of the solids content and of the flow rate. The results are shown graphically in Figures 6, 7, 8 and 9.

The bending strengths were determined in accordance with DIN 51902. They were after pressing the test specimens with a pressure of 1100 bar at max. 200 MPa. Without the spray granulation, only flexural strengths of 180 MPa are obtained.

An overview of the measured values obtained is shown in Table 3. Example 6 - Comparison of continuous dispersion with Baten dispersion

The advantage of the combined process with continuous dispersion in a bead mill with <1 min residence time and subsequent spray drying compared to the two-hour batch dispersion in a drum mill with subsequent subsequent spray granulation lies in the even better material values of the products obtained. Higher debris and tamped densities mean that the granules can be processed better because they have a better mold filling behavior than powders with a low bulk density. This can be recognized by the bending strength of the pellets after carbonation, which are also increased. The measurement results are shown in Table 4 below. In addition, continuous operation is more economical because of the higher space / time yield. In addition, continuous operation is easier to automate.

Table 1

Example 7 - Comparison of the condenser used It was investigated how the liquefiers NSF and lignin sulfonate affect product properties. By using NSF as a liquefier, the obtained characteristics of spray granules and moldings produced therefrom can still be improved compared to the use of lignosulfonate. The results are shown in Table 2.

Table 2

Example 8 - Binder

The influence of the binder on the products obtained according to the invention was investigated. By spray granulation without binder addition comparable product values of the products can be achieved as with binder addition. The results are shown in the following Table 3.

Table 3: Influence of the binder on the material properties

Example 9 - angle of repose

The angle of repose was determined on the basis of DIN 53468. For the determination, 60 g of sample material were poured out of a funnel doben = 140 mm and _diameter = 10 mm, height = 140 mm, and the forming cone was measured, where D stands for diameter. This results in an angle of repose of the spray granulated material from 18.9 to 22.1 ° and the CARBOSINT ^® - starting material of about 41 °.

Example 10 - Grain size distribution

The particle sizes were measured by means of laser diffraction spectrometry and the d ₅₀ value of the particle size distribution. The results are shown below.

CARBOSINT starting material ^β: d ₅₀ = 7-8 microns CARBOSINT ^® granulate: d ₅₀ = 60-100 microns

Claims

claims 1. Continuously feasible process for the preparation of a Halbkokspulvers with high bulk density and flowability, characterized in that 5 a sinterable carbon powder (semi-coke) dispersed in water with the addition of at least one binder and at least one condenser, wherein the I carbon content of the dispersion to at least 50 wt.%, .0 Based on the mass of the dispersion, and the Zeta potential of the dispersion to less than -50 mV, I subjecting the dispersion to homogenization and stabilization .5 by continuous wet grinding with a residence time in the mill of less than 3 minutes, and the homogeneous dispersion is spray-dried. 10 2. The method according to claim 1, characterized in that one uses as a plasticizer produced with formaldehyde polymer of naphthalenesulfonic acid and as a binder PEG 2000 to 12,000. ! 5 3. The method according to claim 1 or 2, characterized in that one carries out the wet grinding continuously in a ball mill. _; 4. The method according to any one of claims 1 to 3, characterized .0 characterized in that adjusting the pH of the Disnersion to at least 7. 5. Aqueous dispersion containing at least 50% by weight of semi-coke powder, 0.5 to 2% by weight of binder and 0.5 to 5% by weight of plasticizer, the weight percentages being 5 of the binder and the Verflüssigeranteils refer to the mass of the solid of the dispersion. 6. Semi-coke granules, obtainable by the process according to , one of claims 1 to 4, characterized by a .0 bulk density according to DIN 51705 of at least 0.52 g / cm ³ and a tamped density according to DIN 51916 of at least 0.59 g / cm ³ , 7. Semi-coke granules according to claim 6, characterized by .5 a particle size d ₅₀ of 60 to 100 microns. 8. Use of a Halbkokspulvers, obtainable according to one of claims 1 to 4 or according to any one of claims 6 or 7, for the production of carbon and ! 0 Graphite ceramics with a density after the Further processing in presses and after carbonization at a temperature of up to 1100 ⁰ C of at least 1.65 g / cm ³ and a bending strength of at least 160 MPa. : 5